1. Field of the Invention
The present invention relates generally to the field of electronic system testing. More particularly, the present invention relates to methods and apparatuses for testing the signal paths in electronic systems.
2. Related Art
Various techniques for testing signal paths in an electronic system are known, including the use of Time Delay Reflectometry (TDR), Frequency Domain Reflectometry (FDR), and Standing Wave Reflectometry (SWR). These conventional techniques are similar in that a test device injects a test signal and then observes the characteristics of a reflected signal reflected to the test device. The reflected signal characteristics are affected by discontinuities in the electrical properties of the signal path being tested. This allows characterization of the signal path, for example, the location of faults (such as open and short circuits), detecting mismatches, or measuring wire length.
These conventional techniques suffer from a number of disadvantages that make them cumbersome and expensive to implement. FDR requires two connections to the signal path being tested, most typically implemented by use of directional couplers, allowing the separation of the injected test signal from the reflected signal. These directional couplers are bulky and expensive. Directional couplers are also limited in frequency range, which limits the type of test signals that can be injected.
Although SWR can be used without directional couplers, SWR is typically limited to providing a pass/fail characterization of the signal path. In SWR techniques, the frequency spacing at which the reflected signal reaches minimums is determined to obtain a rough estimate of the location of a problem. SWR does not perform well when there are multiple problems in the signal path. Isolation of the actual location of the problem along the signal path is difficult to determine accurately with FDR or SWR.
TDR techniques avoid the problem of requiring two connections by injecting a time-limited pulse as the test signal. Confusion between the test signal and reflected signal is thus avoided because they are separated in time. Although TDR eliminates the complexity associated with the directional couplers, TDR suffers from requiring very precise timing. The accuracy with which TDR can locate a problem is limited by the accuracy by which the time-limited test pulse can be generated and the accuracy that the reflected signal can be measured. Hence, accurate TDR systems require complex and expensive timing circuitry. Furthermore, TDR cannot be used with signal paths that are designed to support a narrow range of frequencies because the TDR pulse is a wide-bandwidth signal.